The microbial sulfur cycle is one of the great biological cycles of our world, and it underlies life as we know it. Yet it is not very well studied because there are few tools for quantifying the redox state of the element. X-ray absorption spectroscopy is a powerful tool for distinguishing different forms of sulfur in oils and coals, where the predominant forms of sulfur are generally reduced species, at least in the native forms. We are extending the technique to biological systems, paying particular attention to aqueous sulfur species, and to those likely to be of relevance to the microbial sulfur cycle. We have studied the metabolic forms of sulfur in a range of phototrophic and chemoautotrophic bacteria, grown in the laboratory and collected from the field, with the aim of developing a tool that will allow assessment of the metabolic status of microbial populations collected from the field with a minimum of sample manipulation.